How do L1 visa holder obtain green card?

You may be eligible for an employment-based, first-preference visa if you have an extraordinary ability, are an outstanding professor or researcher, or are a multinational executive or manager. Multinational executive or manager may obtain EB1 visa if 

You have been employed outside the United States in the 3 years preceding the petition for at least 1 year by a firm or corporation and you are seeking to enter the United States to continue service to that firm or organization. Your employment have been outside the United States in a managerial or executive capacity and with the same employer, an affiliate, or a subsidiary of the employer.

Your petitioning employer is a U.S. employer. Your employer have been doing business for at least 1 year, as an affiliate, a subsidiary, or as the same corporation or other legal entity that employed you abroad.

EB1 visa shares the same criteria as L1 visa. Therefore, as long as a person is eligible for L visa, obtaining green card is matter of time. In other word, L1 visa holder is qualified for EB1 green card.